Project a. A comprehensive investigation of major aspects of spinal ganglion development in the chick embryo: Establishment of birthdates of the two populations of neurons in brachial and lumbar ganglia; periods of proliferation and neuron degeneration; counts of neurons, mitotic and degenerating cells in selected stages during these periods. These data will give a picture of the dynamics of attainment of final population size and a basis for b. Project b. The experimental part will be concerned with two aspects. One, the possible role of NGF as a trophic and maintenance agent in normal spinal ganglion development. Injection of labelled NGF into limbs which have acquired sensory innervation will show whether NGF can be transported retrogradely to limb ganglia and, furthermore, whether NGF made accessible by this route will increase proliferation and reduce degeneration numerically. The latter point has not been investigated as yet.